Well that's the issue here I think.
If we have a screen which is 10 pixels wide then do we have pixels 0,1,2,3,4,5,6,7,8,9 - which is 10 pixels and 9 "gaps" or
do we have pixels 0,1,2,3,5,6,7,8,9,10 - which is 11 pixels and 10 "gaps".
Is the width (or height) the number of pixels or the number of "gaps".
If we have one pixel then are we saying it is width 0 or width 1? If width 1 then the first example is correct for a width of 10. If it is width 1 then the second example is right.
I think it's only a problem when you use canvas instructions; and maybe I am worrying too much. ;-)